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Contract No. DE-AC36-99-GO10337 

J. Adelstein and B. Sekulic 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-36583 

Performance and Reliability of 

a 1-kW Amorphous Silicon 

Photovoltaic Roofing System
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Performance and Reliability of a 1-kW Amorphous Silicon 

Photovoltaic Roofing System 

J. Adelstein and B. Sekulic 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 

ABSTRACT

The National Renewable Energy Laboratory (NREL)

has been monitoring the performance of a 1- kWAC United 

Solar Systems Corporation (USSC) roofing system over

the 6-year period from October 1998 to September 2004.

This paper will investigate the performance and reliability 

of this system. The annual degradation and seasonal

fluctuation of the system’s power output are calculated

using the PVUSA power rating regression model. The 

system performance is also examined using the additional

performance parameters of yield, reference yield, and 

performance ratio, which allows for a somewhat less

complicated data collection and analysis. The data

indicate that the system has exhibited stable performance

over time, with an overall degradation rate comparable to

that found in crystalline silicon systems. 

Introduction 

This paper describes the performance, based on

various performance parameters, of the 1.2-kWDC USSC 

roofing system being monitored at NREL over the 6-year

period from October 1998 to September 2004. The PV 

roofing system consists of 72 triple-junction amorphous

silicon (a-Si) SHR-17 shingles / modules manufactured by

United Solar Systems Corporation (USSC), which perform

both as conventional roofing shingles and as a solarelectric array. The 12”-wide by 86.5”-long shingles are

arranged into two 612-W sub-arrays consisting of 6 strings 

of 6 shingles / modules. Each sub-array is mounted on a

south-facing simulated attic / roof at tilt angle from the

horizontal of 40° (NREL’s approximate latitude). The

system is connected to the utility grid through a 4-kW

Trace Engineering SW4048 inverter. 

Experimental in-situ data for the USSC system, as 

well as wind speed data from NREL’s Reference

Meteorological and Irradiance System (RMIS), are

recorded by Campbell Scientific data acquisition systems

(DAS). Measurements are made every 5 s and stored as

15-min averages for the USSC system and 1-min 

averages for the RMIS wind speed data. Data collected for

the USSC system include: plane-of-array (POA) 

irradiance, DC voltage, DC current, AC power, and 

ambient temperature. DC power is calculated by the DAS

by multiplying the DC voltage and DC current. 

System Description 

The PV shingle array consists of 72 USSC SHR-17

shingle modules divided into two sub-arrays. Each shingle 

module provides a peak rated power output of 17.3 W (2.0

amps at 8.5 volts) at 1000 W/m2

. Each sub-array consists

of 6 parallel strings of 6 PV shingles connected in series

and is mounted on a south-facing simulated attic / roof at

tilt angle from the horizontal of 40° (NREL’s approximate

latitude). The two sub-arrays are joined in a combiner box

and connected to the utility grid through a 4-kW Trace 

Engineering SW4048 inverter. 

The DAS system is based on a Campbell Scientific

21XL datalogger. System AC and DC voltages are

measured with Ohio Semitronics, Inc. (OSI), model VT8

variable-frequency voltage transducers. System currents 

are measured with OSI model CTL Hall effect current

transducers coupled with model CTA signal conditioners.

AC power is measured with an OSI model PC8 variablefrequency watt transducer. Irradiance is measured with a

Kipp and Zonen CM11 pyranometer. Ambient

temperature is measured with a Campbell Scientific 107

thermistor temperature probe, and wind speed is 

measured with a Campbell Scientific 03001 Wind Sentry

Wind Set. 

The system installation was completed on June 30, 

1998, and data acquisition started on July 1, 1998.

However, due to an incorrect pyranometer calibration 

factor, data prior to October 22, 1998, was found to be

invalid. Hence, the dataset used for this paper includes

data from October 1998 through September 2004. There 

have been no reliability problems related to the shingles / 

modules during the time period of this study. 

PVUSA power rating analysis

The PVUSA rating system is based on the

assumption that array current primarily depends on

irradiance and that array voltage primarily depends on 

array temperature—which, in turn, depends on array 

temperature, ambient temperature, and wind speed [2]. 

Measured data can be used to calculate a best-fit

correlation to the following equation: 

P = IPOA (a + bIPOA + cTamb + dW) , 

1
where: 

P = power (AC or DC), kW 

 IPOA = POA irradiance, W/m2

 Tamb = ambient temperature, oC 

W = wind speed, m/s 

a,b,c,d = regression coefficients derived

 from operational data 

One-month blocks of 15-min average system

performance and meteorological observations were used 

to obtain the regression coefficients above. Because the

PVUSA model performance is poor at low irradiances, the

POA irradiance inputs to the regression equation were

restricted to IPOA > 800 W/m2

. The measured power was

restricted to PDC > 800 W and PAC > 700 W, in an attempt 

to filter out data that were collected when there was snow

covering the array.

After determining the regression coefficients, we

found the PVUSA power rating for the system for each

one month block by normalizing to PVUSA test conditions

(PTC), where PTC conditions are defined as 1000 W/m2

POA irradiance, 20o

C ambient temperature, and 1 m/s 

wind speed. 

National Instrument’s LabVIEW programs originally

developed at NREL by Keith Emery and modified for this

paper were used to combine the USSC data with the 

RMIS wind speed data and to perform the linear

regression. LabVIEW’s general linear-fit function was used

to determine the set of linear coefficients using the least 

chi-square method. Monthly power ratings are then plotted

to show the system performance vs. time. 

Other System Performance Indices 

Marion et al [3] present three parameters that can

define the overall performance of a PV system with

respect to energy production and the solar resource.

These parameters, defined in the International 

Electrotechnical Commission (IEC) Standard 61724 [5], 

are the final PV system yield (Yf), reference yield (Yr), and 

performance ratio (PR). 

The values for Yf, Yr, and PR are calculated as 

follows: 

Yf = EAC / Po (kWh/kW or hours) (1) 

Yr = Irr / ref Irr (hours) (2) 

PR = Yf/Yr (dimensionless) (3) 

where: 

EAC = system net AC energy output in kWh 

Po = system rated power in kW 

Irr = total plane of array irradiance in kWh/m2

ref Irr = reference irradiance (usually 1 kW/m2) 

The final yield (Yf) represents the number of hours 

that the PV array would need to operate at its rated power 

to produce that amount of energy. The reference yield (Yr) 

represents the equivalent amount of hours necessary for

the array to receive the reference irradiance. The

performance ratio (PR) is thus the ratio of the measured

system energy to the hypothetical energy that would be

produced had the system been operating at its rated 

power under the reference irradiance. It indicates the 

energy output that may be realized under typical field

conditions. By normalizing with respect to irradiance, the

PR quantifies the overall effect of losses on the rated

output due to inverter inefficiency, wiring mismatch, and

other losses due to the DC-to-AC power conversion, 

module reflection losses, and soiling losses [3]. 

The same one-month blocks of 15-min average

system performance and meteorological observations

were used to calculate and plot the monthly performance

ratios for the USSC system. Choosing the performance

ratio versus the PVUSA power rating as a performance

metric simplifies both the data collection, by eliminating 

the need to measure and record the ambient temperature

and wind speed, and analysis, by eliminating the

regression model. 

Results and Discussion

Figure 1 depicts the PVUSA AC and DC power 

ratings of the USSC system vs. time from October 1998

through September 2004 for irradiances greater than 800

W/m2

 and system power greater than 700 

WAC and 800 WDC.

2
800.0

850.0

900.0

950.0

1000.0

1050.0

1100.0

1150.0

Sep-98 Mar-99 Sep-99 Mar-00 Sep-00 Mar-01 Sep-01 Mar-02 Sep-02 Mar-03 Sep-03 Mar-04 Sep-04

Time

Power Rating (W)

DC Power AC Power

AC

-10.8 W per year

-1.09% / yr

DC

-10.2 W per year

-0.985% / yr

Figure 1: USSC Roof - PVUSA Power Rating vs Time

(Irr >800 W/m2 and P > 800 W (P > 700W AC)).

Linear fits to the power rating curves indicate that 

the USSC roof system has degraded at a rate of 10.2 W

(0.985%) per year DC and 10.8 W (1.09%) per year AC

over the 6-year time period from October 1998 through

September 2004. Amorphous silicon modules typically 

experience a light-induced performance degradation on

initial exposure due to the Staebler-Wronski effect [4]. In

a paper summarizing performance characterization of aSi modules from several manufacturers, Sandia

National Laboratories found that previously unexposed

a-Si modules showed an initial rapid degradation in

power over the first 6 months and reached a “stabilized”

power level, about 20% below the initial (1st day) power

after about one year. Seasonal oscillation is usually 

±4% from the “stabilized” level [1]. Because the shingles

/ modules had been deployed for several months before

accurate data acquisition began, the initial rapid 

degradation characteristic of a-Si modules was not 

observed; however, the system demonstrates the typical

seasonal oscillating power output also characteristic of

a-Si modules, i.e., higher power output in the warmer

summer months. Yearly DC seasonal variation for this

system was found to average +4.9% and –5.4% from 

the yearly mean power rating for the five complete years

of data available (1999 through 2003). 

The system performance ratio (PR) is plotted in 

Fig. 2, along with a scaled version of the AC PVUSA

power-rating plot for comparison. A linear fit for the PR 

for the 6-year time period from September 1998 through

September 2004 indicates a degradation rate of 1.13% 

per year, which corresponds well with the degradation

rate of the PVUSA AC power rating of –1.09%. The 

positive yearly seasonal oscillation averaged 4.6%, 

similar to that of the PVUSA method; however, the

negative value averaged –7.8%, somewhat greater than

that found with the PVUSA method. 

Note that the typical winter power-rating decline 

does not appear to have occurred over the 2003 / 2004

winter season when using the PVUSA method. Upon

further investigation of the data, we noted that for two of 

the winter months—in particular, November 2003 and

February 2004—daily high temperatures for a few days

were in the –150

 to –200C range. The average

temperature recorded by RMIS between 1994 and 2004

is 1.70

C in February and 4.90C in November. It is 

possible that such a large extrapolation from 200

C, 

which is the standard for PTC, yielded questionable

results. 

3
0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

Sep-98 Mar-99 Sep-99 Mar-00 Sep-00 Mar-01 Sep-01 Mar-02 Sep-02 Mar-03 Sep-03 Mar-04 Sep-04

Time

Performance Ratio (PR)

Performance Ratio Scaled PVUSA AC Power Rating

PR degradation = -1.13%/yr

Figure 2: USSC Roof System Performance Ratio vs. Time. 

Conclusions

The performance monitoring of the USSC a-Si triplejunction SHR-17 roofing system at NREL over a 6-year

period has shown the shingles / modules to have excellent 

reliability and degradation trends comparable to those for 

crystalline Si modules. The degradation rate was

determined to be 0.985% per year (DC) and 1.09% per

year (AC) by the PVUSA method and 1.13% (AC) by the

performance ratio (PR) method. Season variation was

found to be about 10.5% (PVUSA method) and 12.5% (PR

method). 

References

1. D. King, J. Kratochvil, and W. Boyson.

“Stabilization and Performance Characteristics of 

Commercial Amorphous-Silicon PV Modules,” 

Proc. 28th IEEE PVSC, 2000, p.1446-1449. 

2. C.M. Whitaker T.U. Townsend, “Application and

Validation of a New PV Performance

Characterization Method,” Proc. 26th IEEE PVSC, 

1997, p.1253-1256. 

3. B. Marion, et al., “Performance Parameters for 

Grid-Connected PV Systems,” Proc. 31st IEEE

PVSC, 2005 (in press). 

4. D.L. Staebler and C.R. Wronski, “Reversible

Conductivity Changes in Discharge-Produced

Amorphous Si,” Appl. Phys. Lett. 31, 292–294 

(1977). 

5. IEC, “Photovoltaic system performance

monitoring guidelines for measurement, data

exchange and analysis, IEC Standard 61734”, 

Geneva, Switzerland, 1998.

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Performance and Reliability of a 1-kW Amorphous Silicon 

Photovoltaic Roofing System 

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14. ABSTRACT (Maximum 200 Words)

The National Renewable Energy Laboratory (NREL) has been monitoring the performance of a 1- kWAC United Solar 

Systems Corporation (USSC) roofing system over the 6-year period from October 1998 to September 2004. This 

paper will investigate the performance and reliability of this system. The annual degradation and seasonal fluctuation 

of the system’s power output are calculated using the PVUSA power rating regression model. The system 

performance is also examined using the additional performance parameters of yield, reference yield, and 

performance ratio, which allows for a somewhat less complicated data collection and analysis. The data indicate that 

the system has exhibited stable performance over time, with an overall degradation rate comparable to that found in 

crystalline silicon systems. 

15. SUBJECT TERMS

PV; performance and reliability; power rating regression model; crystalline silicon 

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